Phenylaminyl Cations in the Liquid and Gas-Phase Phototransformation of Protonated Azobenzene

Data on the spectroscopy of photoexcitation oftrans-to-cisisomerization and fragmentation of protonatedtrans-azobenzene (ABH(+)) in an inert gas atmosphere are analyzed. Results from gas-phase isomerization are compared to ones from a liquid-phase reaction in aqueous sulfuric acid that results in the formation of cyclic benzo[c]cinnoline. There is no cyclization upon gas-phasetrans-to-cisisomerization in a rarefied nitrogen medium atT= 300 K, due to the high rate of inversecis-to-transisomerization. It is shown that transformations in both phases proceed with the participation of phenylaminyl type cationstrans- andcis-ABH(+), which are chromogens. Photofragmentation oftrans-ABH(+)in an atmosphere of cryogenically cooled helium (T= 40 K) proceeds with the formation of fragment phenyl and phenyl diazonium cations and the participation of phenyl amine cations. Mechanisms of photofragmentation are proposed, including intermediate acts with the participation of H atoms formed by dissociation of the NH group. A fast attack of H atoms along the phenyl ring bonded to the NH group results in the breaking of the C-N bond, eliminating the possibility oftrans-to-cisisomerization in a cryogenic helium atmosphere.